1) Field of the Invention
The present invention relates generally to grain moisture sensors. More specifically, the present invention relates to an improved grain moisture sensor for combines.
2) Related Art
Grain moisture sensors have been used in combines, particularly in precision agriculture applications. Continuous or instantaneous grain moisture readings allow an operator to observe the moisture of the grain as it is being harvested. In conjunction with a GPS unit, a moisture sensor can be used to provide moisture mapping. In addition, moisture sensors are used in yield monitoring applications. When used in combination with a grain flow sensor, the moisture sensor information is often used to calculate the number of dry bushels in a field and the number of bushels per acre based on the number of wet bushels and the moisture content.
Moisture sensors in combines are commonly mounted in one of two places. The first of these places is in the grain tank auger. The grain tank auger is also known as the loading auger in a combine. There are a number of problems with mounting the moisture sensor in this location. The first is that in order to mount the moisture sensor the flighting of the loading auger must be removed. With removed flighting, material can build up which requires the operator to clean the sensor. If the moisture sensor is not kept clean, readings may be inaccurate or the moisture sensor may be inoperable.
A further problem with mounting the moisture sensor in the loading auger of a combine is the lag time or delay encountered when measuring moisture. When the moisture sensor is mounted in the loading auger position, moisture sensor readings are not taken until the grain is actually in the loading auger of the combine. Therefore, grain must travel up the elevator and fill the sump of the transition housing before the auger is able to deliver grain to the sensor and a moisture measurement can be taken. This deficiency frustrates the use of a moisture sensor in precision agriculture applications, making it more difficult to correctly associate a particular field location with a particular grain moisture.
A further problem with mounting grain moisture sensors in a loading auger is that such a moisture system does not provide for determining when there is sufficient grain present for a grain moisture measurement. Grain moisture sensors usually include capacitive plates. The volume between the plates must be covered before an accurate grain moisture measurement can be made. A moisture sensor that is not filled with grain is not accurately measuring the moisture of the grain. Therefore, this inability to know when the capacitive plate is covered can result in erroneous grain moisture measurements.
Another location that has been used to mount grain moisture sensors is on the side of the clean grain elevator. The clean grain elevator mounting location is thought to provide a steadier flow of grain. Further, the clean grain elevator location may avoid causing accelerated wear of the auger assembly and does not obstruct grain flow in the manner which the loading auger location may. Despite these improvements, a number of problems remain with mounting a moisture sensor on the side of the clean grain elevator in a combine. One problem relates to the slow cycle time of the moisture sensor. In a low flow condition which is not uncommon in grain harvesting, the sensor can be extremely slow to fill. For example, it may take up to four minutes to fill the sensor. Therefore, the number of moisture sensor readings is reduced and the moisture sensor data is insufficient for providing accurate measurements for moisture maps, yield determinations, and other purposes.
A further problem with mounting moisture sensors on the side of the clean grain elevator relates to the sensitivity of this mounting location in the presence of side slopes. It is not uncommon for a combine to be operating on a hill or slope. When the combine is operated on a slope such that the grain flow is directed away from the moisture sensor inlet, it is nearly impossible to fill the grain moisture sensor with sufficient grain to make a moisture determination.
A further problem with mounting moisture sensors on the clean grain elevator relates to grain leaks. When mounted on the side of the clean grain elevator, any grain leaks that occur result in the leaking grain spilling on the ground, as the grain leaks are not contained.
Another problem in grain moisture sensing relates to the sensor cell. Typically, the sensor cell consists of a parallel plate capacitor in which the grain serves as the dielectric material. The cell capacitance and therefore the permittivity of the grain between the plates is measured. From this measurement, the moisture of the grain is determined. Normally, these cell designs are not as close to an ideal parallel plate capacitor as desired. In particular, prior art designs for grain moisture sensors for use in combines use cells that are subject to electric field fringe effects. A fringe effect occurs when electric field lines are not both straight and perpendicular to the plates of the parallel plate capacitor. These fringe effects produce an uncontrollable influence on the measurements from material other than grain that is close to the cell but outside of the cell. Another problem with cell designs is that they do not produce uniformly dense electric field lines between the parallel plates. The nonuniform electric field density creates the problem of unequal sensitivity to grain throughout the cell. Thus the measurements of the moisture of the grain within the cell are not as accurate as desired in these respects.
Another problem relating to the prior art relates to the method for measuring cell capacitance. Measuring the capacitance of a cell filled with grain is a traditional way of obtaining grain moisture. There are two common prior art methods for measuring cell capacitance. The first method is to sense the changes in frequency of a variable oscillator that uses cell capacitance as one of its frequency determining elements. The second method is to excite the cell capacitance with a signal having a known frequency and to measure the absolute value of the resulting cell current, usually with a bridge type of circuit and a peak detector, and then to calculate the capacitance of the cell. Both of these methods tend to be dependent on grain cell construction and are sensitive to noise, changes in circuit characteristics, and parasitic effects. The first method also has the problem of poor control of frequency, especially as moisture varies. Both of these methods are also single dimensional, lacking the ability to measure both the dielectric and the loss properties of the grain. Therefore numerous problems remain with this type of sensing.
The combination of the dielectric and loss parameters is known as the complex permittivity. Complex permittivity is an intrinsic, frequency dependent material property. The knowledge of the grain's complex permittivity at more than one frequency has been found to be a part of advanced moisture level assessment as has been demonstrated by USDA studies. Despite this observation, problems remain.